1. Field of the Invention
The present invention is related to a noise reducing apparatus, and particularly to a noise reducing apparatus of a cyclo-fan that cancels a whistling noise generated by the cyclo-fan.
2. Description of the Prior Art
In general, a cyclo-fan may preferably be used for the purpose of reducing the size of an appliance because the suction and discharge directions of the air flowing therethrough are opposite to each other. Furthermore, the cyclo-fan is widely used as an air circulating means or a cooling means in various appliances including an air conditioner because the amounts of air increase in proportion to the axial length thereof.
As is well known, the air conditioner is an appliance that either absorbs heat from a definite space (in a cooling mode) or discharges heat to a definite space (in a heating mode), by using refrigerant as the working fluid in the refrigerating cycle comprising a compressor, an outdoor heat exchanger, an expansion valve, and an indoor heat exchanger.
FIG. 1 is a schematic cross sectional view showing a conventional indoor unit of an air conditioner.
Referring to FIG. 1, the indoor unit of an air conditioner conventionally comprises an indoor heat exchanger 115 diagonally extended across a housing 110, an air inlet 112 formed in the wall of the housing 110 lying over the indoor heat exchanger 115, an air outlet 113 arranged at the lower-front side of the housing 110, and a cyclo-fan arranged in the vicinity of the air outlet 113 lying under the indoor heat exchanger 115.
In the afore-mentioned configuration, relatively warm air (in a cooling mode) or cool air (in a heating mode) in a definite space flows into the housing 110 through the air inlet 112 by the action of the cyclo-fan 111 while the air conditioner is in operation. The heat of this air is then exchanged with that of the cool refrigerant (in a cooling mode) or the hot refrigerant (in a heating mode) flowing into the indoor heat exchanger 115, and then discharged through the air outlet 113, thereby decreasing or increasing the room temperature.
A stabilizer 114 is secured next to the air outlet 113 in the housing 110 for efficient discharge of the air circulated by the cyclo-fan 111. The narrower the spacing between the stabilizer 114 and the cyclo-fan 111, the greater the amount of air discharged.
On the other hand, the cyclo-fan 111 generates a whistling noise at a specific frequency value of NZ, which is determined by the product of the rotations N per second (i.e. rps) and the number Z of blades 111a of the cyclo-fan 111.
FIG. 2 is a wave form showing the frequency characteristics of a whistling noise generated by a conventional cyclo-fan.
Assuming that the rotation rate N of the cyclo-fan 111 is 20 revolutions per second (1200 rpm) and the number Z of blades 111a of the cyclo-fan 111 is 35, the pressure level of the whistling noise abruptly increases at a frequency of 700 Hz as shown in FIG. 2. Furthermore, the pressure level of the whistling noise becomes higher as the spacing between the cyclo-fan 111 and the stabilizer 114 becomes narrower, which acts as a restrictive factor when attempting to increase the amount of discharged air.
On the other hand, the overall noise generated by the indoor unit of an air conditioner is also caused by mechanical vibrations of the cyclo-fan 111 itself, by vibrations in the air path system, and by vibrations in a refrigerant pipe line or the like. An apparatus for reducing this overall noise is disclosed in Japanese Patent Laid-Open No. 4-281125 (Oct. 6, 1992). The disclosed apparatus includes two microphones for collecting the overall noise, two amplifiers for amplifying the overall noise collected by the microphones in an electrical manner, a main controller for producing an interference sound signal having a reverse phase and same amplitude (same sound pressure) as the overall noise, and two speakers for generating the interference sound cancel-out the overall noise. However, the disclosed apparatus has a problem in that it requires complicated circuits that include an additional signal analysis circuit because it is the overall noise being eliminated, thereby raising the manufacturing cost of the air conditioner.